The present invention relates to a system for managing the data areas of storage devices as virtual data areas.
In recent years, the data amount stored into a storage device has been increased exceedingly. Moreover, the storage capacity of the storage device itself, and the number of storage devices, file servers, or the like which are to be connected to a SAN (: Storage Area Network) have also been increased. As a result, there have appeared the following various problems: The complication in management of the storage areas or the like that have had large capacities now, the complication in management of the storage devices set up at distributed locations, the load concentration on the storage devices, and the like. At present, with the objective of solving these problems, the research and development of a technology referred to as “virtualization” is being carried out.
As is described in the white paper “Virtualization of Disk Storage” (WP-0007-1), which Evaluator Group, Inc. had published in September 2000, the virtualization technology is basically classified into three types.
The first type is as follows: The respective servers coupled to a SAN have shared information for managing the storage areas of storage devices coupled to the SAN. In addition, each server accesses the storage devices, using each volume manager that each server has.
The second type is a system configuration referred to as “In-band Virtualization”. In this configuration, a management server manages, collectively as virtual data areas (which, hereinafter, will be referred to as “virtual volumes”), the storage areas of the storage devices coupled to the SAN. Furthermore, the management server receives an access request to the storage devices from each server, thereby accessing the storage areas of the storage devices under the control and then sending back the result to each server.
The third type is a system configuration referred to as “Out-band Virtualization”. In this configuration as well, the management server collectively manages the storage areas of the storage devices coupled to the SAN. However, when the management server receives the access request to the storage devices from each server, the management server sends back, to each server, position information on a storage area where the data has been actually stored. Then, based on the sent-back information, each server accesses the storage area of the storage devices.
The mainstream structure of a storage device is now a structure where a magnetic disk circular-plate is used as the recording medium. When the magnetic disk circular-plate is used as the recording medium, the following factors become problems: A time needed for the data seek on the magnetic disk, and a time needed for the data transfer from the magnetic disk.
In view of this situation, the data readahead scheme has been implemented. In this scheme, a cache memory is provided inside the storage device. Moreover, data that a host will request, especially data that will be read out consecutively (i.e., sequential data), is predicted. Furthermore, the predicted data is read out onto the cache memory in advance. The present technique has been described in a thesis “An analytic behavior model for disk drives with readahead caches and request reordering” published in “ACM SIGMETRICS '98”, pp. 182-191. This data readahead conceals the above-described factors, i.e., the time needed for the data seek on the magnetic disk and the time needed for the data read-out from the magnetic disk, thereby allowing an enhancement in the data read-out performance. The present technique will be referred to as “data readahead processing”.